Last year, I gradually purchased the key components for a new transistor PA: the enclosure, the transformer, the 4x MRF150 PA PCB, and a set of bandpass filters from a military transmitter. The filters are switched by relays, and their rating appears to be sufficient for high power.
The switching will be controlled in parallel with the antennas by a band decoder.
Cooling will be two-point: the entire assembly will be cooled by a fan on the rear panel, and heat from the power transistors will be dissipated through a copper plate to a large aluminum heat sink. The power supply will be stabilized and of classic design with a toroidal transformer.